The development of computer hard disk drives demands ever increasingly higher track density, lower acoustic noise, and better reliability under shock and vibrational disturbances. The undesirable characteristics of the currently used ball beating spindles, such as high non-repetitive runout, large acoustic noise, and high resonance frequencies due to bearing defect, impose severe limitation on the drive's capacity and performance.
The use of a non-contact bearing, such as a hydrodynamic bearing ("HDB"), may overcome the aforementioned limitation. The full film lubrication of a fluid bearing displays significantly lower non-repetitive runout and acoustic noise, and its higher damping provides better resistance to external shock and vibration. One example of a disk drive spindle motor including a HDB and centrifugal-capillary seals is found in the present inventor's (with other co-inventors) U.S. Pat. No. 5,423,612 entitled: "Hydrodynamic Bearing and Seal", the disclosure thereof being incorporated herein by reference.
The deployment of the HDB system in a hard disk drive environment requires that the lubricant be securely sealed inside of the bearing structure under all operating and non-operating conditions in order to prevent performance degradation of the bearing and contamination in the drive. At the same time, the bearing system needs to be easily manufacturable in order to satisfy cost requirements. As explained below in connection with FIG. 1, these requirements often come into conflict with each other and have heretofore resulted in compromised HDB spindle designs.
Prior approaches in the design of lubricant seals for self-contained hydrodynamic bearing units include surface tension or capillary seals and/or traps, ferromagnetic seals, flow recirculation passages, spiral or herringbone pumping grooves and global flow recirculation of lubricant driven by the centrifugal force and pumping grooves resulting from relative rotation of the components comprising the bearing unit.
Capillary taper seals have been shown to be effective when the bearing unit is at rest (except in response to shock forces which causes the lubricant's surface to break down and lubricant to be lost as droplets). However, capillary sealing force is relatively weak compared to the dynamic pressure build-up inside of the bearing as well as to potential imbalanced pumping as may result from manufacturing tolerances.
Ferromagnetic seals are generally vulnerable to leakage under thermal expansion conditions. On the other hand, pumping grooves have been shown to result in undesirable ingestion of ambient air during operation. Flow recirculation passages, either for localized lubricant flow, or for global flow throughout the structure of the bearing unit, involve considerable manufacturing difficulty and resultant high prime cost of the hydrodynamic bearing unit.
To be effective within a hard disk drive environment, the HDB system is typically configured to include radial hydrodynamic bearing regions formed between a shaft and a sleeve, and axial or thrust bearing regions formed between an annular disk or facing radial surfaces of the shaft and sleeve combination. One exemplary prior art example is presented in FIG. 1. Therein, a HDB system within a height-reduced, cantilevered spindle assembly for a hard disk drive includes a base 11, a bearing shaft 12 suitably secured within an opening in the base 11 as by press fitting and/or an anaerobic adhesive. Alternatively, the HDB system may be assembled as a separate component to a mounting flange, which is in turn mounted within an opening of the base 11. The particular mounting arrangement of the shaft 12 and the base 11 is conventional, and not a part of the present invention.
A sleeve 14 fits over the shaft in a precise arrangement. A disk hub 16 is secured to the sleeve 14 and supports e.g. 1-4 data storage disks in conventional fashion, the disks (and spacers) being retained between a lower flange 17 of the hub 16 and a disk clamp secured to the top of the hub in conventional fashion (the disks, spacers and clamp not being shown in FIG. 1 ). A DC brushless spindle motor includes a multi-coil armature assembly 20 secured to an annular wall 21 of the base 11. A permanent magnet ring 22 and a ferromagnetic flux-return ring 24 complete the brushless motor in this cantilevered motor design. A thrust plate 30 is secured to the shaft 12 e.g. by press-fitting or a screw, and defines two HDB surfaces 32 and 34 between a top plate 40 and a recessed radial wall of the sleeve 14, respectively. Radial bearings 36 and 38 are defined at spaced-apart locations of the, shaft 12 and the sleeve 14, as shown in FIG. 1, for example.
The HDB patterns are precision-formed by suitable forming techniques such as ball-forming for the radial bearings 36 and 38, e.g. as described in commonly assigned, copending U.S. patent application Ser. No. 08/353,171 by Shuo-Hao Chen, filed on Dec. 8, 1994, and entitled: "Method for Making Precision Self-Contained Hydrodynamic Bearing Assembly", the disclosure thereof being incorporated herein by reference. The HDB patterns at the thrust plate 32 may be conventionally formed as by etching or coining techniques.
Of principal interest in connection with the present invention, the prior HDB assembly 10 includes a lower axial capillary seal 42 formed between an inwardly tapered portion of the shaft 12 and a surrounding cylindrical wall portion of the sleeve 14 at a location below the radial bearing 38. Such a seal is shown and more completely discussed in U.S. Pat. No. 4,726,693 to Anderson et al., entitled: "Precision Hydrodynamic Bearing", the disclosure thereof being incorporated herein by reference. One significant drawback of this prior approach is that droplets may be released from the lubricant free surface which resides in the capillary seal 42 because of a shock force or other disturbance occurring while the motor 10 is at rest and settle or form at a location 43 at the lower juncture of the shaft 12 and sleeve 14. Later, when power is applied to the motor 10, resultant relative rotation between the sleeve 14 and shaft 12 creates a centrifugal force which drives the droplet along an annular gap 44 between the sleeve 14 and the annular wall 21 of the base, and results in leakage at the outer juncture 46 of the annular wall 21 and the sleeve 14. Droplets could also form during bearing operation, and centrifugal force-driven leakage similar to that described above could occur. The present invention overcomes this leakage problem.
Another significant drawback of the prior art is that the seal volume in the capillary seal 42 is relatively small compared with the total lubricant volume inside of the bearing unit because the capillary seal 42 is located at the inner diameter of the bearing unit. Under condtions of thermal expansion and/or variations of the lubricant filling tolerance, lubricant leakage can occur at the seal 42. The present invention also overcomes this drawback.
A hitherto unsolved need has remained for an improved HDB unit having seal which overcomes limitations including high prime cost and leakage/loss of lubricant.